Abstract: A UV reactor comprises a reactor body shaped to define longitudinally extending inner and outer reactor chambers. One or more passages located at a first longitudinal end of the inner reactor chamber provide fluid communication between the inner and outer reactor chambers. The outer reactor chamber surrounds a portion of the inner reactor chamber that includes the first longitudinal end. The UV reactor also comprises: an inlet/outlet in fluid communication with the outer reactor chamber; and an outlet/inlet in fluid communication with the inner reactor chamber. The outlet/inlet is located at a second longitudinal end of the inner reactor chamber. A cap connected to the reactor body houses one or more UV radiation emitters optically oriented to direct UV radiation into the inner reactor chamber. An inner fluid-flow cross-section of the inner reactor chamber may be greater than an outer fluid flow cross-section of the outer reactor chamber.